The field of integrated photonics, in particular the integration of photonics into established silicon chip technology, has been a very active field of research for some years and promises great practical benefit in the combination and fusion of electronic and optical data transmission and data processing. Nano-photonic systems having high complexity and a multiplicity of functions can thus be implemented in an extremely small space and can be produced industrially. In this case, waveguide structures play an important part in particular for the optical connection of integrated photonic systems among one another (chip-chip connections) and for connection within a photonic system (on-chip connections).
However, the optical connections between individual photonic systems prove again and again to be a significant technological hurdle for commercial implementation. Thus, both with regard to the positioning accuracy and with regard to the quality or optical quality of such optical connections, requirements are made which can be achieved only with difficulty in the manufacture of large scale integrated circuits. Optical connections for example on the basis of conventional optical fibers which are coupled directly to the chip in any case require a very large number of work steps which are not automatable at all or are automatable only with difficulty, and lead to comparatively low integration densities of chip-chip connections. This in turn leads to high production costs and unit costs.
The technology of the optical connections for integrated photonic systems is thus of crucial importance for the development of photonic circuits. For this purpose, in the past a range of different approaches were put forward for improving the production of photonic connection waveguides in integrated photonic systems. In particular, there were various approaches for integrating optical connections into integrated circuits (ICs) on the basis of CMOS technology which are based on different physical effects. These also include the utilization of an internal photoemission effect for photodetection or light detection.
The term photodetectors generally denotes electronic components which convert light into an electrical signal on the basis of the photoelectric effect or exhibit an electrical resistance dependent on the incident radiation. In this case, the functioning is based on the absorption of light in the form of photons and the subsequent separation of the charge carriers generated by light. Conventional photodiodes are based on the principle that, by means of the absorption of the photons, electrons are raised from the valence and into the energetically higher conduction band, for which purpose the energy of the individual photon must correspond at least to the band gap of the irradiated semiconductor. In this case of linear interaction, however, a large detection area is needed in order to increase the efficiency of the photodiodes. Therefore, conventional photodiodes, for example on the basis of germanium, allow comparatively low integration densities of chip-chip connections.
Very recently there were various approaches for overcoming the disadvantages of conventional photodetectors with utilization of internal photoemission—the field of plasmons.
Surface plasmons are electromagnetic surface waves which are coherently coupled to charge carrier density fluctuations and are bound to interfaces between a metal and an insulator/semiconductor. Analogously to the photon, the quantum of a light wave, the plasmon denotes the quantum of a charge carrier density wave passing through a plasma. In the sense of wave-particle duality, however, plasmon denotes not only the quantum of the wave field but also the continuous charge carrier density wave as a whole. On account of their hybrid character on the basis of electrons and photons, surface plasmons can be used for transmitting information, the advantages residing in the small spatial extent of plasmonic components. The small extent enables low capacitances and thus short reaction times to external electric fields. Surface plasmons are not subject to the diffractive limits of wave mechanics and are distinguished by an amplified electromagnetic field, as a result of which an amplified interaction between light and matter, in particular in the form of nonlinear interaction, occurs. This nonlinearity is fundamentally comparable with that of two-photon absorption, which exhibits a quadratic dependence on the light intensity (power).
The plasmon at a metal-semiconductor interface decomposes as a result of absorption in the metal, high-energy electrons, so-called “hot electrons”, being generated precisely at the interface with the semiconductor. Said hot electrons are able to overcome the potential barrier between semiconductor and metal, which leads to a light-induced charge separation and hence a measurable current. This process is known as internal photoemission. In this case, the potential barrier can be overcome either directly or else in the form of quantum mechanical tunnel effects. The probability of the charge carriers overcoming the potential barrier is determined by an exponential function of the barrier form, e.g. width and height, and of the charge carrier energy.
In order to utilize internal photoemission for light detection, various metal-semiconductor geometries were proposed in the past. By way of example, M. W. Knight et al., Science, Vol. 332 (6030), pp. 702-704 (2011), describe photodetection using active optical antennas, wherein use is made of nanoantennas embedded in silicon in the form of rectangular gold wires. Furthermore, photonic waveguides coated with metals, as described for example in M. Casalino et al., Optics Express, Vol. 21 (23), pp. 28072-28082 (2013), are used as photodetectors.
These approaches attempted to position the non-absorbing counterelectrode as far away as possible from the absorbing electrode, in order to generate as little power loss as possible in the counterelectrode. However, this is associated with long charge carrier drift times. Fundamental problems of these approaches were low quantum efficiencies of less than one percent in the case of nanoantennas on account of poor coupling to the electromagnetic wave, and a small electrical bandwidth, which is approximately 1 GHz in the case of the above-described waveguides in the low GHz range.